Method and a device for detecting flash gas

ABSTRACT

A method and a device for detecting flash gas in a vapor-compression refrigeration or heat pump system comprising a compressor, a condenser, an expansion device, and an evaporator interconnected by conduits providing a flow path for a refrigerant, by determining a first rate of heat flow of a heat exchange fluid flow across a heat exchanger of the system and a second rate of heat flow of the refrigerant across the heat exchanger, and using the rates of heat flow for establishing an energy balance from which a parameter for monitoring the refrigerant flow is derived, to thereby provide early detection of flash gas with a minimum number of false alarms.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates byreference essential subject matter disclosed in international PatentApplication No. PCT/DK2003/000468 filed on Jul. 3, 2003 and DanishPatent Application No. PA 2002 01072 filed on Jul. 8, 2002.

FIELD OF THE INVENTION

The present invention relates to a method and a flash gas detectiondevice for detecting flash gas in a vapour-compression refrigeration orheat pump system comprising a compressor, a condenser, an expansiondevice, and an evaporator interconnected by conduits providing a flowpath for a refrigerant.

BACKGROUND OF THE INVENTION

In vapour-compression refrigeration or heat pump systems the refrigerantcirculates in the system and undergoes phase change and pressure change.In the system a refrigerant gas is compressed in the compressor toachieve a high pressure refrigerant gas, the refrigerant gas is fed tothe condenser (heat exchanger), where the refrigerant gas is cooled andcondensates, so the refrigerant is in liquid state at the exit from thecondenser, expanding the refrigerant in the expansion device to a lowpressure and evaporating the refrigerant in the evaporator (heatexchanger) to achieve a low pressure refrigerant gas, which can be fedto the compressor to continue the process.

However, in some cases refrigerant in the gas phase is present in theliquid refrigerant conduits caused by boiling liquid refrigerant. Thisrefrigerant gas in the liquid refrigerant conduits is denoted “flashgas”. When flash gas is present at the entry to the expansion device,this seriously reduces the flow capacity of the expansion device by ineffect clogging the expansion device, which impairs the efficiency ofthe system. The effect of this is that the system is using more energythan necessary and possibly not providing the heating or coolingexpected, which for instance in a refrigerated display cabinet for shopsmay lead to warming of food in the cabinet, so the food must be thrownaway. Further the components of the system will be outside normaloperating envelope. Because of the high load and low mass flow ofrefrigerant when flash gas is present, the compressor may be subject tooverheating, especially in the event that misty oil in the refrigerantis expected to function as lubricant the compressor will undergo alubrication shortage causing a compressor seizure.

Flash gas may be caused by a number of factors: 1) the condenser is notable to condense all the refrigerant because of high temperature of theheat exchange fluid, 2) there is a low level of refrigerant because ofinadequate charging or leaks, 3) the system is not designed properly,e.g. if there is a relatively long conduit without insulation from thecondenser to the expansion device leading to a reheating and possiblyevaporation of refrigerant, or if there is a relatively large pressuredrop in the conduit leading to a possible evaporation of refrigerant.

A leak in the system is a serious problem, as the chosen refrigerant maybe hazardous to the health of humans or animals or the environment.Particularly some refrigerants are under suspicion to contribute in theozone depletion process. In any event the refrigerant is quite expensiveand often heavily taxed, so for a typical refrigerated display cabinetfor a shop recharging the system will be a considerable expense.Recently a shop having refrigerated display cabinets lost half of therefrigerant in the refrigeration system before it was detected that therefrigeration system had a leak, and recharging of the system was anexpense of 75,000 dkr, approximately 10,000 $.

A known way to detect flash gas is to provide a sight glass in a liquidconduit of the system to be able to observe bubbles in the liquid. Thisis labour and time consuming and further an observation of bubbles maybe misleading, as a small amount of bubbles may occasionally be presenteven in a well functioning system.

Another way is to indirectly detect flash gas by triggering an alarmwhen the expansion device is fully open, e.g. in the event that theexpansion device is an electronic expansion valve or the like. In thiscase a considerable number of false alarms may be experienced, as afully open expansion device may occur in a properly functioning systemwithout flash gas.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method for early detection offlash gas with a minimum number of false alarms.

This object is met by a method comprising the steps of determining afirst rate of heat flow of a heat exchange fluid flow across a heatexchanger of the system and a second rate of heat flow of therefrigerant across the heat exchanger, and using the rates of heat flowfor establishing an energy balance from which a parameter for monitoringthe refrigerant flow is derived. Hereby it is possible to monitor therefrigerant flow without direct measurement using a flow meter. Suchflow meters are expensive and may further restrict the flow.

According to an embodiment, the heat exchanger is the evaporator, whichis the ideal component.

According to an alternative or additional embodiment, the heat exchangeris the condenser.

As will be appreciated by the skilled person the first rate of heat flowof the heat exchange fluid can be established in different ways, butaccording to an embodiment the method comprises establishing the firstrate of heat flow by establishing a heat exchange fluid mass flow and aspecific enthalpy change of the heat exchange fluid across the heatexchanger.

According to an embodiment, the method comprises establishing the heatexchange fluid mass flow as a constant based on empirical data or ondata obtained under faultless operation of the system.

According to an embodiment, the method comprises establishing thespecific enthalpy change of the heat exchange fluid across the heatexchanger based on measurements of the heat exchange fluid temperaturebefore and after the heat exchanger.

The second rate of heat flow of the refrigerant may by determined byestablishing a refrigerant mass flow and a specific enthalpy change ofthe refrigerant across the heat exchanger.

The refrigerant mass flow may be established in different ways,including direct measurement, which is, however, not preferred.According to an embodiment, the method comprises establishing therefrigerant mass flow based on a flow characteristic of the expansiondevice, and the expansion device opening passage and/or opening period,and an absolute pressure before and after the expansion device, and ifnecessary any subcooling of the refrigerant at the expansion deviceentry.

The specific enthalpy difference of the refrigerant flow may beestablished based on registering the temperature and pressure of therefrigerant at expansion device entry and registering the refrigerantevaporator exit temperature and the refrigerant evaporator exit pressureor the saturation temperature of the refrigerant at the evaporatorinlet.

A direct evaluation of the refrigerant mass flow is possible, but mayhowever be subject to some disadvantages, e.g. because of fluctuationsor variations of the parameters in the refrigeration or heat pumpsystem, and it is hence preferred that the method comprises establishinga residual as difference between the first rate of heat flow and thesecond rate of heat flow.

To further reduce the sensibility to fluctuations or variations ofparameters in the system and be able to register a trend in therefrigerant mass flow at an early time, the method may compriseproviding a fault indicator by means of the residual, the faultindicator being provided according to the formula:

$S_{\mu_{1},i} = \{ \begin{matrix}{{S_{\mu_{1},{i - 1}} + s_{i}},{{{{when}\mspace{14mu} S_{\mu_{1},{i - 1}}} + s_{\mu_{1},i}} > 0}} \\{0,{{{{when}\mspace{14mu} S_{\mu_{1},{i - 1}}} + s_{\mu_{1},i}} \leq 0}}\end{matrix} $

where s_(μ) ₁ _(,i) is calculated according to the following equation:

$S_{\mu_{1},i} = {{- {k_{1}( {r_{i} - \frac{\mu_{0} + \mu_{1}}{2}} )}}\mspace{14mu}{where}}$

r_(i): residual

k₁: proportionality constant μ₀: first sensibility value μ₁: secondsensibility value.

According to a second aspect the invention regards a flash gas detectiondevice, which comprises means for determining a first rate of heat flowof a heat exchange fluid flow across a heat exchanger of the system anda second rate of heat flow of the refrigerant across the heat exchanger,and using the rates of heat flow for establishing an energy balance fromwhich a parameter for monitoring the refrigerant flow is derived, thedevice further comprising evaluation means for evaluating therefrigerant mass flow, and generate an output signal.

According to an embodiment of the device, the means for determining thefirst rate of heat flow comprises means for sensing heat exchange fluidtemperature before and after a heat exchanger.

According to an embodiment of the device, the means for determining thesecond rate of heat flow comprises means for sensing the refrigeranttemperature and pressure at expansion device entry, and means forsensing the refrigerant temperature at evaporator exit, and means forestablishing the pressure at the expansion device exit or the saturationtemperature.

According to an embodiment of the device, the means for establishing thesecond rate of heat flow comprises means for sensing absoluterefrigerant pressure before and after the expansion device and means forestablishing an opening passage or opening period of the expansiondevice.

To provide a robust evaluation means, the evaluation means may comprisemeans for establishing a residual as difference between a first value,which is made up of the mass flow of the heat exchange fluid flow andthe specific enthalpy change across a heat exchanger of the system, anda second value, which is made up of the refrigerant mass flow and thespecific refrigerant enthalpy change across a heat exchanger of thesystem.

To be able to evaluate a trend in the output signal, the device mayfurther comprise memory means for storing the output signal and meansfor comparing said output signal with a previously stored output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in detail withreference to the drawing, where

FIG. 1 is a sketch of a simple refrigeration system or heat pump system,

FIG. 2 is a schematic log p, h-diagram of a cycle of the systemaccording to FIG. 1,

FIG. 3 is a sketch of a refrigerated display cabinet comprising therefrigeration system according to FIG. 1,

FIG. 4 is a sketch showing a part of the refrigerated display cabinetaccording to FIG. 3,

FIG. 5 is a diagram of a residual in a fault situation, and

FIG. 6 is a diagram of a fault indicator in the fault situationaccording to FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following reference will be made to a simple refrigerationsystem, although the principle is equally applicable to a heat pumpsystem, and as understood by the skilled person, the invention is in noway restricted to a refrigeration system.

A simple refrigeration system is shown in FIG. 1. The system comprises acompressor 5, a condenser 6, an expansion device 7 and an evaporator 8interconnected by conduits 9 in which a refrigerant is flowing. The modeof operation of the system is well known and comprises compression of agaseous refrigerant from a temperature and pressure at point 1 beforethe compressor 5 to a higher temperature and pressure at point 2 afterthe compressor 5, condensing the refrigerant under heat exchange with aheat exchange fluid in the condenser 6 to achieve liquid refrigerant athigh pressure at point 3 after the condenser 6. The high-pressurerefrigerant liquid is expanded in the expansion device 7 to a mixture ofliquid and gaseous refrigerant at low pressure at point 4 after theexpansion device. In this simple example, the expansion device is anexpansion valve, but other types of expansion devices are possible, e.g.a turbine, an orifice or a capillary tube. After the expansion device,the mixture flows into the evaporator 8, where the liquid is evaporatedby heat exchange with a heat exchange fluid in the evaporator 8. In thissimple example, the heat exchange fluid is air, but the principleapplies equally to refrigeration or heat pump systems using another heatexchange fluid, e.g. brine, and further the heat exchange fluid in thecondenser and the evaporator need not be the same.

FIG. 2 is a log p, h-diagram of the refrigeration system according toFIG. 1, showing pressure and enthalpy of the refrigerant. Referencenumeral 10 denotes the saturated vapour curve, 11 the saturated liquidcurve and C.P. the critical point. In the region 12 to the right ofsaturated vapour line 10, the refrigerant is hence superheated gas,while in the region 13 to the left of the saturated liquid line 11, therefrigerant is subcooled liquid. In the region 14, the refrigerant is amixture of gas and liquid. As can be seen, at point 1 before thecompressor, the refrigerant is completely gaseous and during thecompression, the pressure and temperature of the refrigerant is raised,so at point 2 after the compressor, the refrigerant is a superheated gasat high pressure. The refrigerant leaving the condenser 6 at point 3should be completely liquid, i.e. the refrigerant should be at a stateon the saturated liquid curve 11 or in the region 13 of subcooled liquidrefrigerant. In the expansion device 7, the refrigerant is expanded to amixture of liquid and gas at a lower pressure at point 4 after theexpansion device 7. In the evaporator 8, the refrigerant evaporates atconstant pressure by heat exchange with a heat exchange fluid so as tobecome completely gaseous at the exit of the evaporator at point 1.

If, as indicated by point 3′, the refrigerant entering the expansiondevice 7 is a mixture of liquid and gas, the previously mentioned flashgas, then the refrigerant mass flow is restricted as previouslymentioned and the cooling capacity of the evaporator 8 of therefrigeration system is significantly reduced. Further, but lesssignificant the available enthalpy difference in the evaporator 8 isreduced, which also reduces the cooling capacity.

FIG. 3 shows schematically a refrigerated display cabinet comprising arefrigeration system. Refrigerated display cabinets are i.a. used insupermarkets to display and sell cooled or frozen food. The refrigerateddisplay cabinet comprises a storage compartment 15, in which the food isstored. An air channel 16 is arranged around the storage compartment 15,i.e. the air channel 16 run on both sides of and under the storagecompartment 15. After travel through the air channel 16, an air stream17, shown by arrows, enters a cooling zone 18 over the coolingcompartment 15. The air is then again lead to the entrance to the airchannel 16, where a mixing zone 19 is present. In the mixing zone 19 theair stream 17 is mixed with ambient air. Thereby air, which has enteredthe storage compartment or somehow escaped into the surroundings, issubstituted. In the air channel 16 is provided a blower device 20, whichcan be made up of one or more fans. The blowing device 20 ensures thatthe air stream 17 can be moved in the air channel 16. The refrigerateddisplay cabinet comprises part of a simple refrigeration system asoutlined in FIG. 1, as an evaporator 8 of the system is placed in theair channel 16. The evaporator 8 is a heat exchanger exchanging heatbetween the refrigerant in the refrigeration system and the air stream17. In the evaporator 8 the refrigerant takes up heat from the airstream 17, which is cooled thereby. The cycle of the refrigerationsystem is as described with regard to FIGS. 1 and 2, and with thenumerals used therein.

As mentioned, it is highly advantageous in a refrigeration or heat pumpsystem to be able to detect flash gas, i.e. the presence of gas at theexpansion device entry. The effect of flash gas is a reduced mass flowthrough the expansion device when compared to the mass flow in thenormal situation of solely liquid refrigerant at the expansion deviceentry. When the refrigerant mass flow in the refrigeration system isless than the theoretical refrigerant mass flow provided solely liquidphase refrigerant at the expansion device entry, this difference is anindication of the presence of flash gas. The refrigerant mass flow maybe established by direct measurement using a flow meter. Such flowmeters are, however, relatively expensive, and may further restrict theflow creating a pressure drop, which may in itself lead to flash gasformation, and certainly impairs the efficiency of the system. It istherefore preferred to establish the refrigerant mass flow by othermeans, and one possible way is to establish the refrigerant mass flowbased on the principle of conservation of energy or energy balance ofone of the heat exchangers of the refrigeration system, i.e. theevaporator 8 or the condenser 6. In the following reference will be madeto the evaporator 8, but as will be appreciated by the skilled personthe condenser 6 could equally be used.

The energy balance of the evaporator 8 is based the following equation:{dot over (Q)}_(Air)={dot over (Q)}_(Ref)  (1)

where {dot over (Q)}_(Air) is the heat removed from the air per timeunit, i.e. the rate of heat flow delivered by the air, and {dot over(Q)}_(Ref) the heat taken up by the refrigerant per time unit, i.e. therate of heat flow delivered to the refrigerant.

The basis for establishing the rate of heat flow of the refrigerant({dot over (Q)}_(Ref)) i.e. the heat delivered to the refrigerant pertime unit is the following equation:{dot over (Q)} _(Ref) ={dot over (m)} _(Ref)(h _(Ref,Out) −h_(Ref,In))  (2)

where {dot over (m)}_(Ref) is the refrigerant mass flow. h_(Ref,Out) isthe specific enthalpy of the refrigerant at the evaporator exit, andh_(Ref,In) is the specific enthalpy of the refrigerant at the evaporatorentry. The specific enthalpy of a refrigerant is a material and stateproperty of the refrigerant, and the specific enthalpy can be determinedfor any refrigerant. The refrigerant manufacturer provides a log p,h-diagram of the type according to FIG. 2 for the refrigerant. With theaid of this diagram the specific enthalpy difference across theevaporator can be established. For example to establish h_(Ref,In) withthe aid of a log p, h-diagram, it is only necessary to know thetemperature and the pressure of the refrigerant at the expansion deviceentry (T_(Ref,In) and P_(Con), respectively) . Those parameters may bemeasured with the aid of a temperature sensor or a pressure sensor.Measurement points and parameters measurement points and parameters ofthe evaporator 8 and the refrigeration system can be seen in FIG. 4,which is a sketch showing a part of the refrigerated display cabinetaccording to FIG. 3.

To establish the specific enthalpy at the evaporator exit, twomeasurement values are needed: the temperature at evaporator exit(T_(Ref,out)) and either the pressure at the exit (P_(Ref, out)) or thesaturation temperature (T_(Ref,sat)) . The temperature at the exit ofthe evaporator 8 can be measured with a temperature sensor, and thepressure at the exit can be measured with a pressure sensor.

Instead of the log p, h-diagram, it is naturally also possible to usevalues from a chart or table, which simplifies calculation with the aidof a processor. Frequently the refrigerant manufacturers also provideequations of state for the refrigerant.

The mass flow of the refrigerant may be established by assuming solelyliquid phase refrigerant at the expansion device entry. In refrigerationsystems having an electronically controlled expansion valve, e.g. usingpulse width modulation, it is possible to determine the theoreticalrefrigerant mass flow based on the opening passage and/or the openingperiod of the valve, when the difference of absolute pressure across thevalve and the subcooling (T_(V,in)) at the expansion valve entry isknown. Similarly the refrigerant mass flow can be established inrefrigeration systems using an expansion device having a well-knownopening passage e.g. fixed orifice or a capillary tube. In most systemsthe above-mentioned parameters are already known, as pressure sensorsare present, which measure the pressure in condenser 6. In many casesthe subcooling is approximately constant, small and possible toestimate, and therefore does not need to be measured. The theoreticalrefrigerant mass flow through the expansion valve can then be calculatedby means of a valve characteristic, the pressure differential, thesubcooling and the valve opening passage and/or valve opening period.With many pulse width modulated expansion valves it is found forconstant subcooling that the theoretical refrigerant mass flow isapproximately proportional to the difference between the absolutepressures before and after and the opening period of the valve. In thiscase the theoretical mass flow can be calculated according to thefollowing equation:{dot over (m)} _(Ref) =k _(exp)·(P _(con) −P _(Ref,out))·OP  (3)

where P_(Con) is the absolute pressure in the condenser, P_(Ref,out) thepressure in the evaporator, OP the opening period and k_(Exp) aproportionality constant, which depend on the valve and subcooling. Insome cases the subcooling of the refrigerant is so large, that it isnecessary to measure the subcooling, as the refrigerant flow through theexpansion valve is influenced by the subcooling. In a lot of cases it ishowever only necessary to establish the absolute pressure before andafter the valve and the opening passage and/or opening period of thevalve, as the subcooling is a small and fairly constant value, andsubcooling can then be taken into consideration in a valvecharacteristic or a proportionality constant.

Similarly the rate of heat flow heat of the air ({dot over (Q)}_(Air)),i.e. the heat taken up by the air per time unit may be establishedaccording to the equation:{dot over (Q)} _(Air) ={dot over (m)} _(Air)(h _(Air,in) −h_(Air,out))  (4)

where {dot over (m)}_(Air) is the mass flow of air per time unit,h_(Air,in) is the specific enthalpy of the air before the evaporator,and h_(Air,out) is the specific enthalpy of the air after theevaporator.

The specific enthalpy of the air can be calculated based on thefollowing equation:h _(Air)=1,006·t+x(2501+1.8·t),[h]=kJ/kg  (5)

where t is the temperature of the air, i.e. T_(EVa,in) before theevaporator and T_(EVa,out) after the evaporator. x denotes the absolutehumidity of the air. The absolute humidity of the air can be calculatedby the following equation:

$\begin{matrix}{x = {0\text{,}{62198 \cdot \frac{p_{W}}{p_{Amb} - p_{W}}}}} & (6)\end{matrix}$

Here p_(W) is the partial pressure of the water vapour in the air, andp_(Amb) is the air pressure. p_(Amb) can either be measured or astandard atmosphere pressure can simply be used. The deviation of thereal pressure from the standard atmosphere pressure is not ofsignificant importance in the calculation of the amount of heat per timeunit delivered by the air. The partial pressure of the water vapour isdetermined by means of the relative humidity of the air and thesaturated water vapour pressure and can be calculated by means of thefollowing equation:P _(W) =P _(W,Sat)·RH  (7)

Here RH is the relative humidity of the air and p_(W,Sat) the saturatedpressure of the water vapour. p_(W,Sat) is solely dependent on thetemperature, and can be found in thermodynamic reference books. Therelative humidity of the air can be measured or a typical value can beused in the calculation.

When equations (2) and (4) is set to be equal, as implied in equation(1), the following is found:{dot over (m)} _(Ref)(h _(Ref,Out) −h _(Ref,In))={dot over (m)} _(Air)(h_(Air,In) −h _(Air,Out))  (8)

From this the air mass flow {dot over (m)}_(Air) can be found byisolating {dot over (m)}_(Air):

$\begin{matrix}{{\overset{.}{m}}_{Air} = {{\overset{.}{m}}_{Ref} \cdot \frac{( {h_{{Ref},{Out}} - h_{{Ref},{In}}} )}{( {h_{{Air},{In}} - h_{{Air},{Out}}} )}}} & (9)\end{matrix}$

Assuming faultless air flow this equation can be used the evaluate theoperation of the system.

In many cases it is recommended to register the theoretical air massflow in the system. As an example this theoretical air mass flow can beregistered as an average over a certain time period, in which therefrigeration system is running under stabile and faultless operatingconditions. Such a time period could as an example be 100 minutes.

A certain difficulty lies in the fact that the signals from thedifferent sensors (thermometers, pressure sensors) are subject tosignificant variation. These variations can be in opposite phase, so asignal for the theoretical refrigerant mass flow is achieved, whichprovides certain difficulties in the analysis. These variations orfluctuations are a result of the dynamic conditions in the refrigerationsystem. It is therefore advantageous regularly, e.g. once per minute, toestablish a value, which in the following will be denoted “residual”,based on the energy balance according to equation (1):r={dot over (Q)} _(Air) −{dot over (Q)} _(Ref)

so based on the equations (2) and (4), the residual can be found as:

$\begin{matrix}{r = {{{\overset{\overset{\_}{.}}{m}}_{Air}( {h_{{Air},{In}} - h_{{Air},{Out}}} )} - {{\overset{.}{m}}_{Ref}( {h_{{Ref},{Out}} - h_{{Ref},{In}}} )}}} & (10)\end{matrix}$

where

${\overset{\overset{\_}{.}}{m}}_{Air}$is the estimated air mass flow, which is established as mentioned above,i.e. as an average during a period of faultless operation. Anotherpossibility is to assume that

${\overset{\overset{\_}{.}}{m}}_{Air}$is a constant value, which could be established in the very simpleexample of a refrigerated display cabinet as in FIGS. 3 and 4 having aconstantly running blower.

In a refrigeration system operating faultlessly, the residual r has anaverage value of zero, although it is subject to considerablevariations. To be able to early detect a fault, which shows as a trendin the residual, it is presumed that the registered value for theresidual r is subject to a Gaussian distribution about an average valueand independent whether the refrigeration system is working faultless ora fault has arisen.

In principle the residual should be zero no matter whether a fault ispresent in the system or not, as the principle of conservation of energyor energy balance of course is eternal. When it is not the case in theabove equations, it is because the prerequisite for the use of theequations used is not fulfilled in the event of a fault in the system.

In the event of flash gas in the expansion device, the valvecharacteristic changes, so that k_(Exp) becomes several times smaller.This is not taken into account in the calculation, so the rate of heatflow of the refrigerant {dot over (Q)}_(Ref) used in the equations isvery much larger than in reality. For the rate of heat flow of the air({dot over (Q)}_(Air)), the calculation is correct (assuming a faultcausing reduced air flow across the heat exchanger has not occurred),which means that the calculated value for the rate of heat flow of theair ({dot over (Q)}_(Air)) across the heat exchanger equals the rate ofheat flow of the air in reality. The consequence is that the average ofthe residual becomes negative in the event of flash gas in the expansiondevice.

In the event of a fault causing reduced air flow across the heatexchanger (a defect blower or icing up of the heat exchanger) the massflow of air is less than the value for the mass flow of air

${\overset{\overset{\_}{.}}{m}}_{Air}$used in the calculations. This means that the rate of heat flow of theair used in the calculations is larger than the actual rate of heat flowof the air in reality, i.e. less heat per unit time is removed from theair than expected. The consequence (assuming correct rate of heat flowof the refrigerant, i.e. no flash gas), is that the residual becomespositive in the event of a fault causing reduced air flow across theheat exchanger.

To filter the residual signal for any fluctuations and oscillationsstatistical operations are performed by investigating the followinghypotheses:

1. The average value of the residual r is μ₁ (where μ₁<0). Correspondingto a test for flash gas.

2. The average value of the residual r is μ₂ (where μ₂>0). Correspondingto a test for reduced air flow.

The investigation is performed by calculating two fault indicatorsaccording to the following equations:

1. Test for Flash Gas:

$\begin{matrix}{S_{\mu_{1},i} = \{ \begin{matrix}{{S_{\mu_{1},{i - 1}} + s_{i}},{{{{when}\mspace{14mu} S_{\mu_{1},{i - 1}}} + s_{\mu_{1},i}} > 0}} \\{0,{{{{when}\mspace{14mu} S_{\mu_{1},{i - 1}}} + s_{\mu_{1},i}} \leq 0}}\end{matrix} } & (11)\end{matrix}$

where S_(μ) ₁ _(,i) is calculated according to the following equation:

$\begin{matrix}{{S_{\mu_{1},i} = {{- k_{1}}( {r_{i} - \frac{\mu_{0} + \mu_{1}}{2}} )}}\mspace{11mu}} & (12)\end{matrix}$

where k₁ is a proportionality constant, μ₀ a first sensibility value, μ₁a second sensibility value, which is negative as indicated above.

2. Test for Reduced Air Flow:

$\begin{matrix}{S_{\mu_{2},i} = \{ \begin{matrix}{{S_{\mu_{2},{i - 1}} + s_{i}},{{{{when}\mspace{14mu} S_{\mu_{2},{i - 1}}} + s_{\mu_{2},i}} > 0}} \\{0,{{{{when}\mspace{14mu} S_{\mu_{2},{i - 1}}} + s_{\mu_{2},i}} \leq 0}}\end{matrix} } & (13)\end{matrix}$

where S_(μ) _(2,i) is calculated according to the following equation:

$\begin{matrix}{s_{\mu_{2},i} = {k_{1}( {r_{i} - \frac{\mu_{0} + \mu_{2}}{2}} )}} & (14)\end{matrix}$

where k₁ is a proportionality constant, μ₀ a first sensibility value, μ₂a second sensibility value, which is positive as indicated above.

In equation (11) it is naturally presupposed that the fault indicatorS_(μ) ₁ _(,i), i.e. at the first point in time, is set to zero. F or alater point in time is used S_(μ) ₁ _(,i) according to equation (12),and the sum of this value and the fault indicator S_(μ) ₁ _(,i) at aprevious point in time is computed. When this sum is larger than zero,the fault indicator is set to this new value. When this sum equals or isless than zero, the fault indicator is set to zero. In the simplest caseμ₀ is set to zero. μ₁ is a chosen value, which e.g. establish that afault has arisen. The parameter μ₁ is a criterion for how often it isaccepted to have a false alarm regarding flash gas detection.

Similarly in equation (13) it is naturally presupposed that the faultindicator S_(μ) ₂ _(,i), i.e. at the first point in time, is set tozero. For a later point in time is used S_(μ) ₂ _(,i) according toequation (14), and the sum of this value and the fault indicator S_(μ) ₂_(,i) at a previous point in time is computed. When this sum is largerthan zero, the fault indicator is set to this new value. When this sumequals or is less than zero, the fault indicator is set to zero. In thesimplest case μ₀ can be set to zero. μ₂ is an estimated value, whiche.g. establish that a fault has arisen. The parameter μ₂ is a criterionfor how often is it accepted to have a false alarm regarding the airmass flow.

When for example a fault occurs in that flash gas is present at theexpansion valve entry, then the fault indicator will grow, as theperiodically registered values of the S_(μ) ₁ _(,i) in average is largerthan zero. When the fault indicator reaches a predetermined value analarm is activated, the alarm showing that the refrigerant mass flow isreduced. If a smaller value of μ₁ is chosen, i.e. a more negative value,fewer false alarms are experienced, but there exist a risk of reducingsensitivity for detection of a fault.

The principle of operation of the filtering according to equation (11)and (13) shall be illustrated by means of FIGS. 5 and 6. In FIG. 5 thetime in minutes is on the x-axis and on the y-axis the residual r.Between t=200 and 300 minutes a blower fault was present, which gaverise to a significant rise in the residual. Further in the periodst=1090 to 1147 and t=1455 to 1780, flash gas is present, which can beseen as a significant reduction of the residual to a value of about−10×10⁶. However, as can be seen the signal is subject to quitesignificant fluctuations and variations, which makes evaluationdifficult.

The different fault situations can be seen from FIG. 5, but a betterpossibility of identification is present when monitoring the faultindicators S_(μ) ₁ _(,i) and S_(μ) ₂ _(,i), the behaviour of which canseen in FIG. 6, where the dot-dash line denotes S_(μ) ₁ _(,i) and thecontinuous line denotes the S_(μ) ₂ _(,i). Here the value of the faultindicators S_(μ) ₁ _(,i), S_(μ) ₂ _(,i) is on the y-axis and the time inminutes is on the x-axis. The fault indicator S_(μ) ₂ _(,i) growscontinuously in the period between t=200 and 330 minutes because of theblower fault. An alarm can be triggered when S_(μ) ₂ _(,i) exceeds avalue of e.g. 0.2×10⁹. As can be seen by comparison of FIGS. 5 and 6early detection is possible, especially when using the fault indicator.Similarly the fault indicator S_(μ) ₁ _(,i) rises in the period betweent=1090 to 1147 because of flash gas, then gradually reduces back to zeroand then rises again in the period t=1455 to 1780, when flash gas againis present at the expansion valve entry. The fault indicators S_(μ) ₁_(,i), S_(μ) ₂ _(,i) could be set back to zero, when the refrigerationsystem has been working faultless long enough. In praxis the faultindicators S_(μ) ₁ _(,i), S_(μ) ₂ _(,i) would anyway be set to zero whena fault is corrected.

As can be seen in FIGS. 5 and 6 it is hence possible simultaneously toevaluate the system for reduced air flow and flash gas at the expansiondevice entry by evaluating the fault indicators using the criterions uμ₁and μ₂.

Further by means of the method and device according to the invention, itis possible to gain valuable information about the design of therefrigeration system. Many refrigeration systems are tailor made for thespecific use, e.g. for a shop having one or more refrigerated displaycabinets, and some times these refrigeration systems are not optimal,i.e. because of long conduits, pressure drops because of bends of theconduits or the like, or conduits exposed to heating by the environment.With the method and device it will be possible to detect that therefrigeration system is not optimal, and an expert could be sent for toevaluate the system and propose improvements of the system and/orpropose improvements for future systems.

A further advantage of the device is that it may be retrofitted to anyrefrigeration or heat pump system without any major intervention in therefrigeration system. The device uses signals from sensors, which arenormally already present in the refrigeration system, or sensors, whichcan be retrofitted at a very low price.

In the preceding description a simple example was used to illustrate theprinciple of the invention, but as will be readily understood by theskilled person, the invention can be applied to a more complex systemhaving a plurality of heat exchangers, i.e. more than one condenserand/or more than one evaporator.

1. A flash gas detection device for a vapour-compression refrigerationor heat pump system comprising a compressor, a condenser, an expansiondevice, and an evaporator interconnected by conduits providing a flowpath for a refrigerant, wherein the device comprises: means fordetermining a first rate of heat flow of a heat exchange fluid flowacross a heat exchanger of the system and a second rate of heat flow ofthe refrigerant across the heat exchanger, and using the rates of heatflow for establishing an energy balance from which a residual formonitoring the refrigerant mass flow is derived; and evaluation meansfor evaluating the refrigerant mass flow, and providing an output signalindicating the presence or absence of flash gas, based on the residual,wherein the means for determining the second rate of heat flow usesinputs from means for sensing absolute refrigerant pressure before andafter the expansion device, means for establishing an opening passage oropening period of the expansion device, and means for storing a valuerepresenting a flow characteristic of the expansion device, withoutrequiring measurement of refrigerant temperature at the expansion deviceentry and exit.
 2. The device according to claim 1, wherein the heatexchanger is the evaporator.
 3. The device according to claim 1, whereinthe heat exchanger is the condenser.
 4. The device according to claim 1,wherein the means for determining the first rate of heat flowestablishes a heat exchange fluid mass flow and a specific enthalpychange of the heat exchange fluid across the heat exchanger.
 5. Thedevice according to claim 4, wherein the means for determining the firstrate of heat flow establishes the heat exchange fluid mass flow as aconstant based on empirical data or based on data obtained underfaultless operation of the system.
 6. The device according to claim 4,wherein the means for determining the first rate of heat flow comprisesmeans for sensing heat exchange fluid temperature before and after theheat exchanger.
 7. The device according to claim 1, wherein the meansfor determining the second rate of heat flow establishes the refrigerantmass flow and a specific enthalpy change of the refrigerant across theheat exchanger.
 8. The device according to claim 1, wherein the meansfor determining the second rate of heat flow establishes the refrigerantmass flow based on the flow characteristic of the expansion device, andthe expansion device opening passage and/or opening period, and anabsolute pressure before and after the expansion device, withoutmeasuring subcooling of the refrigerant at the expansion device entry.9. The device according to claim 1, wherein the means for determiningthe second rate of heat flow establishes the specific enthalpydifference of the refrigerant flow based on registering the pressure ofthe refrigerant at the expansion device entry and the refrigerantevaporator exit pressure or the saturation temperature of therefrigerant at the evaporator inlet.
 10. The device according to claim1, wherein the residual is derived as a difference between the firstrate of heat flow and the second rate of heat flow.
 11. The deviceaccording to claim 10, wherein the evaluation means evaluates therefrigerant mass flow by means of a fault indicator S_(μ) ₁ _(,i)provided according to the formula:$S_{\mu_{1},i} = \{ \begin{matrix}{{S_{\mu_{1},{i - 1}} + s_{i}},{{{{when}\mspace{14mu} S_{\mu_{1},{i - 1}}} + s_{\mu_{1},i}} > 0}} \\{0,{{{{when}\mspace{14mu} S_{\mu_{1},{i - 1}}} + s_{\mu_{1},i}} \leq 0}}\end{matrix} $ where s_(μ) ₁ _(,i) is calculated according to thefollowing equation:$s_{\mu_{1},i} = {- {k_{1}( {r_{i} - \frac{\mu_{0} + \mu_{1}}{2}} )}}$where r_(i): residual k₁: proportionality constant μ₀: first sensibilityvalue μ₁: second sensibility value.
 12. The device according to claim11, wherein the evaluation means provides the output signal indicatingthe presence or absence of flash gas according to the formula:${{output}\_{signal}} = \{ \begin{matrix}{{PRESENT},\mspace{14mu}{{{when}\mspace{14mu}\underset{\_}{{the}\mspace{14mu}{fault}\mspace{14mu}{indicator}}\mspace{14mu} S_{\mu_{1},i}} > a}} \\{{{predetermined}\mspace{14mu}{value}};} \\{{ABSENT},\mspace{14mu}{{otherwise}.}}\end{matrix} $
 13. The device according to claim 1, wherein themeans for determining the first rate of heat flow comprise means forsensing heat exchange fluid temperature before and after a heatexchanger.
 14. The device according to claim 1, wherein the means fordetermining the second rate of heat flow establishes the refrigerantheat flow according to the equation:{dot over (Q)} _(ref) =k _(exp)(P _(con) −P _(ref,out))×OP×(h _(ref,out)−h _(ref,in)) where {dot over (Q)}_(ref) is the second rate of heatflow; k_(exp): proportionality constant representing the flowcharacteristic of the expansion device; P_(con): refrigerant pressure inthe condenser; P_(ref,out): refrigerant pressure at the evaporator exit;OP: opening period or opening passage of the expansion device;h_(ref, out): refrigerant enthalpy at the evaporator exit, based onP_(ref,out); and h_(ref,in): refrigerant enthalpy at the evaporatorentry, based on P_(ref,out).
 15. The device according to claim 1,wherein the evaluation means comprises means for deriving the residualas a difference between a first value, which is made up of the mass flowof the heat exchange fluid flow and the specific enthalpy change acrossthe heat exchanger, and a second value, which is made up of the massflow and the specific enthalpy change of the refrigerant across the heatexchanger.
 16. The device according to claim 1, wherein the devicefurther comprises memory means for storing the output signal and meansfor comparing said output signal with a previously stored output signal.17. The device according to claim 1, further comprising means foractivating an alarm based on the output signal indicating presence ofthe flash gas.
 18. A flash gas detection device for a vapour-compressionrefrigeration or heat pump system comprising a compressor, a condenser,an expansion device, and an evaporator interconnected by conduitsproviding a flow path for a refrigerant, wherein the device comprises:means for determining a first rate of heat flow of a heat exchange fluidflow across a heat exchanger of the system and a second rate of heatflow of the refrigerant across the heat exchanger, and using the ratesof heat flow for establishing an energy balance from which a residualfor monitoring the refrigerant mass flow is derived; and evaluationmeans for evaluating the refrigerant mass flow, and providing an outputsignal indicating the presence or absence of flash gas, based on theresidual, wherein the means for determining the second rate of heat flowuses inputs from means for sensing absolute refrigerant pressure beforeand after the expansion device, means for establishing an openingpassage or opening period of the expansion device, and means for storinga value representing a flow characteristic of the expansion device, anddetermines the refrigerant mass flow according to the equation:{dot over (m)} _(ref) =k _(exp)·(P _(con) −P _(ref,out))·OP where {dotover (m)}_(ref): refrigerant mass flow k_(exp): value representing theflow characteristic of the expansion device P_(con): absoluterefrigerant pressure before the expansion device P_(ref,out): absoluterefrigerant pressure after the expansion device OP: opening passage oropening period of the expansion device, without requiring measurement ofrefrigerant temperature at the expansion device entry and exit.
 19. Aflash gas detection device for a vapour-compression refrigeration orheat pump system comprising a compressor, a condenser, an expansiondevice, and an evaporator interconnected by conduits providing a flowpath for a refrigerant, wherein the device comprises: means fordetermining a first rate of heat flow of a heat exchange fluid flowacross a heat exchanger of the system and a second rate of heat flow ofthe refrigerant across the heat exchanger, and using the rates of heatflow for establishing an energy balance from which a residual formonitoring the refrigerant mass flow is derived; and evaluation meansfor evaluating the refrigerant mass flow, and providing an output signalindicating the presence or absence of flash gas, based on the residual,wherein the means for determining the second rate of heat flow usesinputs from means for sensing absolute refrigerant pressure before andafter the expansion device, means for establishing an opening passage oropening period of the expansion device, and means for storing a valuerepresenting a flow characteristic of the expansion device, and theevaluation means provides the output signal indicating the presence ofthe flash gas in case the time average of the residual is less thanzero.